This invention relates to the preparation of treated silica.
It is known that water in a silica composition is harmful in certain applications. For instance, it is conventional to heat silica supports for such catalytic agents as chromium in order to remove water (dehydration). As higher dehydration temperatures are used surface hydroxyl, i.e., --OH groups are also removed, thus releasing additional water and forming.tbd.SiOSi.tbd.(siloxane) structures. This dehydroxylation is generally thought of as a continuation of, or a specific form of, dehydration since water is given off due to the formation of silicon-oxygen-silicon bonds.
Chromium oxide catalysts on silica-containing supports can be used to prepare olefin polymers in a hydrocarbon solution to give a product having excellent characteristics from many standpoints. Supported chromium oxide catalysts can also be used to prepare olefin polymers in a slurry system wherein the polymer is produced in the form of small particles of solid material suspended in a diluent. This process, frequently referred to as a particle-form process, has the advantage of being less complex. However, certain control operations which are easily carried out in the solution process are considerably more difficult in the particle-form process. For instance, in a solution process, control of the molecular weight can be effected by changing the temperature, with lower molecular weight (higher melt flow) being obtained at the higher temperatures. However, in the slurry process, this technique is inherently limited since any effort to increase the melt flow to any appreciable extent by increasing temperature causes the polymer to go into solution and thus destroys this slurry or particle-form process.
One way to achieve a higher melt flow polymer is to heat the support to a higher temperature since the melt index increases with increasing activation temperature due to the removal of progressively more --OH groups up to the point where sintering begins. Since further melt index improvement would be possible if more --OH groups could be removed, it would be desirable to utilize some method other than simple heating to remove --OH groups.